Laminated glass is used for automobile safety glass, glazing materials for public facilities and sports/excercise facilities, partitions and security doors. Laminated glass is prepared by replacing a plurality of inorganic glass plates, or some of them, with organic glass, i.e. synthetic resin plates, and laminating them via interlayer films.
Laminated glass in this invention refers to a plurality of transparent plates, comprising glass plates and polycarbonate resin plates or acrylic resin plates, joined together via interlayer films.
For interlayer films for laminated glass, polyvinylbutyral resin plasticized by the addition of a plasticizer, which has a combination of superior adhesion to glass, tough tensile strength and a high degree of transparency, has been used as the most common material.
However, when an interlayer film of this resin is stored as a roll, it is necessary to store it at 5.degree. C. or lower, or to use releasing paper to prevent blocking, because it shows strong self-adhesion at 10.degree. C. or higher temperatures. Furthermore, butyral resin needs to be adjusted for humidity before the lamination process with glass, and the lamination process requires an autoclave. These complicate the lamination process. Also, when this interlayer film is used to laminate synthetic resin transparent plates, the plasticizer in the interlayer film bleeds into the interface with the synthetic resin plate, thus making the adhesion insufficient and at the same time turning the synthetic resin plate white by erosion.
In order to overcome these shortcomings, modified ethylene-vinyl acetate copolymer (Japanese Patent Publication No. 72-2103) and a laminated glass prepared by heating (hardening) a thermosetting resin composition comprising ethylene-vinyl acetate copolymer and organic peroxide during the lamination process (Japanese Patent Publication No. 90-53381) have been proposed.
However, since the former is an ethylene-vinyl acetate modified with acid, it has shortcomings in that: the acid causes higher hygroscopicity; the adhesive property to glass and synthetic resin plates is poor; and crystallinity lowers transparency (the haze value in particular).
On the other hand, for the latter: the transparency improves because the crystallinity decreases during the thermal modification; room temperature storage is possible; humidity control is not necessary; and lamination can be done without using an autoclave. However, since it utilizes radical generated from the decomposition of organic peroxide, it requires a high process temperature, 130.degree. C. or higher, so that it requires heat resistant dyes when used for ornamental laminated glass, leading to poor workability and higher costs. It also has the shortcoming of thermal deformation when used for synthetic resin plates.
Japanese Laid-open Patent Publication No. 92-362046 describes the addition of a nucleating agent (transparency improvement agent), such as dibenzylidenesorbitol, to thermosetting resin comprising ethylene-vinyl acetate copolymer, hydrocarbon resin and organic peroxide blended together. However, this is not sufficient in terms of impact resistance and adhesive property.
Therefore, no prior art interlayer film described thus far satisfies all the requirements for use for laminated glass, such as lamination process characteristics, transparency and adhesive property.